Deoxygenated blood is being pumped into the left atrium which contains oxygenated blood. Mixing deoxygenated blood with oxygenated blood doesn't seem logical. I know that this amount of deoxygenated blood from the bronchial circulation is a small portion of the total systemic return, but I want to know why this occurs.

In human heart, oxygenated blood goes into left atrium by vena pulmonalis, but deoxygenated blood comes into R atrium, then goes into R ventricle, and finally leaves heart going through arteria pulmonalis. In Chondrychthyes and Osteichthyes(fishes), for example, there is no oxygenated blood going through their heart. In Amphibians, there are two atriums, but one ventricle, but still oxygenated blod goes into left, and deoxygenated into right atrium.Reptiles - only crocodiles have, just like Aves(birds) and Mammalia, two atriums, and two ventricles. And again, oxygenated blood goes into left atrium, deoxygenated into right atrium. So, I don't know where you found that incorrect information

Yes, this is the human heart. Bronchial circulation (feeding the lungs themselves) does indeed dump deoxygenated blood into the left atrium. I know that this is not a significant issue, it just makes me curious. I have found no real answers to explain this.

Mith, I've explored that option. There are some that believe that it is a remnant of the ductus arteriosus, but animals like opossums never have an ductus arteriosus and have the same setup for venous return...